Recently, as electronic instruments have become wireless and portable, non-aqueous electrolyte-based secondary batteries with high capacity and high energy density are practically used as drive sources for the electronic instruments. However, such non-aqueous electrolyte-based secondary batteries are problematic in that they cause a drop in battery capacity during repeated charge/discharge cycles due to the following reasons (1) to (5), and particularly cause a significant drop in battery capacity under high-temperature conditions:
(1) A transition metal contained in a composite oxide for a cathode may be dissolved into a non-aqueous electrolyte and precipitated on an anode, resulting in structural collapse of the composite oxide, or an increase in the interfacial resistance;
(2) The transition metal dissolved out of the cathode grows continuously to cause a minute short circuit between the cathode and the anode;
(3) The transition metal dissolved out of the cathode and precipitated on the anode may function as a catalyst for accelerating decomposition of the non-aqueous electrolyte, thereby generating gas in the battery;
(4) The SEI (Solid Electrolyte Interface) layer of the anode has an increased film thickness during repeated charge/discharge cycles, and interrupts movement of Li+ ions; and
(5) The SEI layer is broken gradually due to the expansion/contraction of an anode active material.
In general, (1) a cathode active material, comprising a lithium-containing metal oxide capable of lithium and/or lithium ion intercalation/deintercalation, reacts with an electrolyte comprising a carbonate solvent and a lithium salt under high-temperature conditions, to increase the resistance in the electrode; and (2) the SEI layer formed on the surface of the anode active material capable of lithium and/or lithium ion intercalation/deintercalation is broken gradually at high temperature due to repeated charge/discharge cycles, so that a poor SEI layer is formed by the carbonate solvent and irreversible reactions (such as Li corrosion) are accelerated. Therefore, a non-aqueous electrolyte-based secondary battery has a problem of a significant drop in the battery quality and efficiency, particularly under high-temperature conditions.
Meanwhile, in the case of a non-aqueous electrolyte-based secondary battery, problems related to safety occur under overcharge conditions due to the following reasons. Cathode active materials, such as a lithium-containing metal oxide, capable of lithium and/or lithium ion intercalation/deintercalation are converted into thermally unstable materials due to the deintercalation of lithium under overcharge conditions. Additionally, the structural collapse of the unstable cathode active material results in an exothermic reaction, and the oxygen liberated from the lithium-containing metal oxide, thereby accelerating the reaction between an electrolyte and a cathode and the combustion reaction of an electrolyte. Such chain reactions accompanied with heat emission finally result in a so-called thermal runaway phenomenon.
Generally, factors affecting the safety of a battery under overcharge conditions include: (1) heat emission due to the oxidation of an electrolyte; and (2) heat emission caused by the structural collapse of a cathode.
When overcharge proceeds, heat emission occurring from the above factors independently or simultaneously causes an increase in the internal temperature of a battery, followed by ignition or explosion of the battery. Thus, batteries show a safety problem upon overcharge.
Meanwhile, ignition and explosion of a battery occur due to the so-called thermal runaway phenomenon, when external physical impact (for example, exposure to high temperature by heating) is applied to the battery while the battery is charged or overcharged, and thus the battery experiences a local short circuit; when a battery is overheated due to the heat emission caused by the reaction of a flammable electrolyte with a cathode active material; and when the oxygen generated from an electrode (particularly, a cathode) accelerates the combustion of an electrolyte.